There have been a number of proposals for the construction of stringed musical instruments, such as guitars, to either allow them to be manufactured more efficiently or to be more stable in terms of their susceptibility to changes in temperature and humidity. Examples of such existing manufacturing techniques include the use of plastic polymers (which can be fibre reinforced) for the construction of both the necks and bodies, and various methods of reinforcing the necks of more traditional wood based designs to control or limit their curvature or bending under the effect of the tension exerted by the strings. The latter reinforcements include steel rods (commonly termed “truss rods”) that include an adjustment mechanism to allow control over the curvature of the neck (and hence the curvature of the fingerboard), plus steel or carbon fibre strips, aluminium extrusions or castings in aluminium or magnesium that simply reinforce the neck without providing a means of adjustment.
Normally, if a truss rod type of construction is used then a steel rod is placed along the axis of the neck of the instrument in such a manner that it is disposed to the rear of the neck (the opposite side from the finger board) in the cross section area of the neck (behind the neutral axis) that experiences tensile forces, (or reduced compressive forces compared to the front part of the neck), when the neck is loaded by the tension from the strings. Typically, the rod is fixed at one end of the neck (usually the body end) and a threaded adjustment mechanism is provided at the other end (along the axis of the rod) that allows the rod to be tensioned by varying amounts. The rod may also be curved in shape and act upon the neck material as it is tensioned to control the neck curvature. The variations in tension generated in the truss rod allow the curvature of the neck (and hence the fingerboard of the instrument) to vary, providing more or less clearance for the elliptical shape of the vibrating strings when the latter are plucked or bowed by the musician.
FIG. 1a is a simplified side elevation view of first type of existing instrument where the neck 1 is effectively a non-adjustable “strong” beam or strut incorporating a fingerboard 6 against which the strings 2 are pressed when played. The beam 1 resists the tension in the strings 2, which are attached to body 4 of the instrument at position 3 and extend to the end of the neck at position 5. The effects of the string tension (which is in effect an offset loading producing both compressive and bending forces) are shown in FIG. 1b, which shows their load/tension produces a neck deflection 8, which results in the fingerboard being curved as shown at position 7.
FIG. 2a shows a similar simplified side elevation view of another type of existing instrument that has strings 2 disposed between anchorage positions 3 and 5 and a fingerboard surface 6 and body 4. A beam 9 is supplemented by a truss rod 10 that is anchored to one end of the neck or body 4 at position 11, and includes a threaded part 13 engaging with a threaded nut 12. FIG. 2b shows the effects of the tension of the strings 2 producing a displacement 8 of the neck, which results in a curvature of the fingerboard as shown in position 7. FIG. 2c shows how changes to the threaded adjustment mechanism 12 and 13 can alter the deflection 14 of the neck and hence the curvature of the fingerboard. In this Figure the effect of tightening the nut 12 can be seen as a reduction in the neck curvature and hence a reduction in the fingerboard curvature). Note that the adjustment mechanism is substantially in line with the axis of the neck of the instrument. Whilst this type of arrangement is practical where the neck of the instrument is made from wood it is much more difficult to incorporate in a neck formed from plastic mouldings. Also, such truss rods do not take any of the compressive loads imparted by the string tension; they actually increase the compressive loads in the neck and this can affect the resonance of the neck or the instrument as a whole.